RF directional couplers

Directional coy piers are devices that will pass signal across one path while passijig a much smaller signal along another path. One of the most common uses of the directional coupler is to sample a RF power signal either for controlling transmitter output power level or for measurement. An example of the latter use is to connect a digital frequency counter to the low-level port and the transmitter and antenna to the straight-through (high-power) ports.

The circuit symbol for a directional coupler is shown in Fig, 11-1. Note rhat there are three outputs and one input- The 1N-ÜUT path is low-loss and is the principal path between the signal source and the load. The coupled output is a sample of the forward path while the isolated showed very low signal. If the IN and OUT are reversed then the roles of the coupled and isolated ports also reversed.

An implementation of this c ircuit using transmission line segments is shown in Fig. 11-2. Each transmission line segment (TL, and TLj) has a characteristic impedance, and is a quarter-wavelength long. The path from port I to port 2 is the low-loss signal direction. If power flows in this direction, then port 3 is the coupled port and port 4 is isolated. If the power flow direction reverses (port 2 to ixjrt 1) then the respective roles of p¡>rt 3 and port 4 are reversed.

For a coupling ratio (port 3/port 4) 15 dB f he val up of coupling capacitance must be:

0 18

id Zn

The coupling ratio is:

where

The bandwidth it> about 12%,

DIRECTIONAL COUPLER

ISOLATED

COUPLED

13-1

Directional coupler circuit, .symbol

PORT 1 INPUT

PORT 2 OUTPUT

POFIT3 COUPLED

PORT 4 ISOLATED

11*2 Transmission line directional coupler.

PORT 1 INPUT

PORT 4 ISOLATED

pcrrx.

11-3 L-C version of directional coupler

Lt rrm

Lt mm

PORT 2 OUTPUT

PORT 3 COUPLED

The circuit shown in Fig. 11-3 is an LC lumped constant version of the transmission lines. This network can be used to replace TLi and TU in Fig. 11-2. The values of the components are :

Figure 11-4 shows a directional coupler used in a lot of RF power meters and VSWR meters. The transmission lines are implemented as printed circuit board tracks. It consists of a main transmission line CTLO between Ports 1 and 2 (the low-loss path) and a coupled line (TLa) to form the coupled and isolated ports. The coupling capacitance (in picofarads) is approximated by 9.399A' (X is in meters) when implemented on G-10 Epoxy fiberglass printed circuit board.

PORT 1 INPUT

pofrre

OUTPUT

PORT 3 COUPLED

11-4 lYanamissiott line directional coupler.

PORT 4 ISOLATED

A reflectometer directional coupler is shown in Fig, 11-5A. This type of directional coupler is at the heart of many commercial VSWR meters and RF power meters used in the HF through low-VHF regions of the spectrum. This circuit is conceptually similar to the previous transmission line but is designed around a toroid transmission line transformer, It consists of a transformer in which the low-loss path is a single-turn primary winding and a secondary wound of enameled wire.

Details of the pick-up sensor are shown in Fig. ! 1 -SB. The secondary is wound around the rim of the toroid in the normal manner, occupying not more than 330* of circumference. A rubber or plastic grommet is fitted into the center hole of the toroid core. The single-turn primary is formed by a single conductor passed once through the hole in the center of the grommet. It turns out the 3/16* o,d. brass tub-ing (the kind sold in hobby shops that cater to model builders) will fit through several standard grommet sizes nicely and will slip-fit over the center conductor of SO-239 coaxial connectors.

TORO ID CORE

Power Meter

11-5 (A) Directional coupler suitable for use in an RF power meter; (B) Physical im pieman tation.

Another transmission line directional coupler is shown in Fig. 11-6, Two short, lengths of RG-58/U transmission line 06*) are passed through a pair of toroid coils. Each coil is wound with 8 to 12 turns of wire. Note tliat the shields of the I wo transmission line segments are grounded only at one end.

Each combination of transmission line and toroid core form a transformer similar to the previous case. These two transformers arc cross-coupled to form the network shown. The XMTR-ANTENNA path is the low-loss path, while (with the signal flow direction shown) the other two coupled poits are for forward and reflected power samples. These samples can be rectified and used to indicate the relative power levels flowing in the forward and reverse directions, T^ken together these indications allow us to calculate VSWRr

Directional couplers are used for RF power sampling in measurement and transmitter control. They can also be used in receivers between the mixer or RF amplifier and the antenna input circuit. This arrangement can prevent the flow of LO signal and mbter products back toward the antenna where they could be radiated and cause electromagnetic interference (EMI) to other devices.

XMTfi FORWARD

Build Directional Coupler

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Responses

  • ida
    How to build a coupler cir uit?
    1 year ago
  • erik
    How to calculate turn ratios in RF couplers?
    11 months ago
  • sirkka
    How to build RF directional coupler?
    2 months ago

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